1. Field of the Invention
The present invention relates to a magnetic sensor using a magnetically biased magnetoelectric transducer element and used for, for example, a bank note identifying apparatus for identifying the denomination of a bank note.
2. Description of the Prior Art
A conventional magnetic sensor for identifying the letters and marks printed in magnetic ink or the like on an object being detected such as a bank note and a bond has a structure shown in FIGS. 17 to 23.
In these drawings, the reference numeral 1 represents a comparatively long sensor case made of a nonmagnetic material such as an aluminum diecast. The sensor case 1 is provided with a plurality of (e.g., 3) vertical through slots as unit receiving slots 1A, and inclined surfaces 1B formed on both sides in the widthwise direction of the upper surface thereof at which a note bank guide plate (not shown) is located.
A detection unit 2 inserted into each of the unit receiving slots 1A is made of a nonmagnetic material such as a plastic material and is mainly composed of a rectangular stem 3 inserted into the unit receiving slot 1A; a permanent magnet 4 with a part thereof buried in the upper portion of the stem 3 and the upper surface and the undersurface thereof magnetized as an S-pole and an N-pole, respectively; a magnetoresistance element 5 as a magnetoelectric transducer element which is magnetically biased by the permanent magnet 4; and terminal pins 6 for connecting the magnetoresistance element 5 to the output terminal, as shown in FIG. 19. The magnetoresistance element 5 is located in the vicinity of the upper opening of the unit receiving slot 1A on the side of the surface over which a bank note passes and outputs a detection signal converted into a voltage in accordance with the presence or absence of magnetic ink on a bank note when the bank note passes over a head cover 8 which will be described later.
The reference numeral 7 denotes a side spacer made of a nonmagnetic material such as plastic and ceramic materials and provided on the stem 3 on both sides in the widthwise direction of the magnetoresistance element 5. The height of the side spacer 7 is so determined as to be slightly higher than the upper surface of the magnetoresistance element 7, thereby preventing the head cover 8 from warping, which may result in the damage of the magnetoresistance element 5 or generation of piezo noise.
The head cover 8 is attached to the upper surface side of the sensor case 1 so as to cover the side spacers 7. The head cover 8 is made of a nonmagnetic metal material such as tungsten, phosphor bronze, titanium and an alloy thereof.
The head cover 8 is composed of a flat portion 8A which has a flat surface so as to guide a bank note and adhesive surfaces 8B formed by bending the right and left side portions of the flat portion 8A into two bend portions, respectively. Each of the adhesive surfaces 8B is bonded with the corresponding inclined surface 1B and side surface of the sensor case 1 with an adhesive and fixed thereto.
When these members are assembled into a magnetic sensor, the detection units 2 are inserted into the respective unit receiving slots IA, and the side spacers 7 are respectively attached to the right and left sides of the permanent magnet 4. Each of the adhesive surfaces 8B is bonded with the corresponding inclined surface 1B and side surface of the sensor case 1 with an adhesive, and thereafter a heated plastic material as a molding material 9 is poured into each unit receiving slot 1A from the undersurface side of the sensor case 1 so as to fix each detection unit 2 with the molding.
In the magnetic sensor having the above-described structure, when a bank note passes over the head cover 8, each of the magnetoresistance elements 5 produces an output voltage in accordance with the presence or absence of magnetic ink on the bank note, and the denomination is identified from the waveform of the output voltage.
Such a conventional magnetic sensor, however, has the following problems.
When a sensor case 1 is produced, a dimensional error is produced not only in the unit receiving slot 1A but also in the stem 3 of the detection unit 2 and the position at which the magnetoresistance element 5 is pasted. As a result, when the detection unit 2 is inserted into the unit receiving slot 1A of the sensor case 1, gaps .DELTA.e.sub.1 and .DELTA.e.sub.2 are produced on both sides of the unit receiving slot 1A, as shown in FIG. 22. If it is assumed that the width of the unit receiving slot 1A is a and the width of the stem 3 is b, the gap .DELTA.e is represented by the following formula (1) ##EQU1##
Therefore, if it is assumed that when a bank note advances in the direction indicated by the arrow 10, as shown in FIG. 23, the detection reference surface of the sensor case 1 is A--A, the normal detection line along which the magnetoresistance element 5 carries out the detection operation is B--B, and the distance between the reference surface A and the detection line B is C, the distance C varies in the range of the gap .DELTA.e with each magnetoresistance element 5. As a result, the position of the detection line B--B varies with each magnetoresistance element 5, thereby disabling the accurate detecting operation, so that the user is inconveniently required to repeat the insertion of the bank note many times.
In addition, in the conventional magnetic sensor, the head cover 8 is bonded with the sensor case 1 merely by fixing the right and left adhesive surfaces 8B of the head cover 8 to the side surfaces of the sensor case 1 with an adhesive. The head cover 8 is only made of one sheet of a nonmagnetic metal material to form the flat portion 8A and the adhesive surfaces 8B consisting of two bent portions.
Therefore, even in the state in which the head cover 8 is adhered and fixed to the sensor case 1, the unfolding force in the direction indicated by the arrow a in FIG. 22 acts on the adhesive surfaces 8B as a peel force, and the head cover 8 is easy to peel off the sensor case 1. Consequently, when the sensor case 1 is bolted to a machine frame or the like, or external impact force is applied to the sensor case 1 in order to incorporate the assembled magnetic sensor shown in FIG. 17 into a bank note identifying apparatus, stress acts on the head cover 8 and the adhesive joints are peeled off due to warping, distortion or the like, thereby disadvantageously removing the head cover 8.